NATURAL SELECTION 230 
of pupil, but destitute of a lens or other optical contrivance. With 
insects it is now known that the numerous facets on the cornea of their 
great compound eyes form true lenses, and that the cones include 
curiously modified nervous filaments. But these organs in the 
Articulata are so much diversified that Miiller formerly made three 
main classes with seven subdivisions, besides a fourth main class of 
aggregated simple eyes. 
When we reflect on these facts, here given much too briefly, with 
respect to the wide, diversified, and graduated range of structure in the 
eyes of the lower animals; and when we bear in mind how small the 
number of all living forms must be in comparison with those which 
have become extinct, the difficulty ceases to be very great in believing 
that natural selection may have converted the simple apparatus of an 
optic nerve, coated with pigment and invested by transparent mem- 
brane, into an optical instrument as perfect as is possessed by any 
member of the Articulate Class. 
He who will go thus far, ought not to hesitate to go one step fur- 
ther, if he finds on finishing this volume that large bodies of facts, 
otherwise inexplicable, can be explained by the theory of modification 
through natural selection; he ought to admit that a structure even as 
perfect as an eagle’s eye might thus be formed, although in this case 
he does not know the transitional states. It has been objected that 
jn order to modify the eye and still preserve it as a perfect instrument, 
many changes would have to be effected simultaneously, which, it is 
assumed, could not be done through natural selection; but as I have 
attempted to show in my work on the variation of domestic animals, 
it is not necessary to suppose that the modifications were all simulta- 
neous, if they were extremely slight and gradual. Different kinds of 
modification would, also, serve for the same general purpose: as 
Mr. Wallace has remarked, “if a lens has too short or too long a 
focus, it may be amended either by an alteration of curvature, or an 
alteration of density; if the curvature be irregular, and the rays do not 
converge to a point, then any increased regularity of curvature will be 
an improvement. So the contraction of the iris and the muscular 
movements of the eye are neither of them essential to vision, but 
only improvements which might have been added and perfected at any 
stage of the construction of the instrument.” Within the highest 
division of the animal kingdon, namely, the Vertebrata, we can start 
from an eye so simple, that it consists, as in the lancelet, of a little 
